Hi,
Support for Windows 8.1 already ended and the installation media is no longer available from Microsoft website.
In case you already upgrade to Windows 10 and it was activated, you may download the Windows 10 from https://www.microsoft.com/en-us/software-download/windows10.
You may also contact your device's manufacturer support and in case Windows 8.1 already installed in your PC, they might be able to give you the recovery media.